Acquired cytopenias are blood disorders in which one or more blood cell types are greatly reduced. Understanding hematopoietic stem and progenitor cell (HSPC) development is critical for the development of new cytopenia treatments. Signal transduction pathways and networks of transcription factors orchestrate hematopoiesis. However, agonizing these pathways to increase blood development comes with the risk of promoting hematological malignancies. MiRNAs have been described as fine-tuners of gene expression as they typically downregulate their targets by less than 50%. We hypothesize that through targeting multiple genes in a signaling or transcriptional network miRNAs have major effects on cell differentiation. However we postulate that modulating miRNA expression will carry lower risk of promoting malignancy due to their modest effects on individual targets. Using overexpression and shRNA experiments, we have published that the miRNA cluster mirn23a, which codes for 3 pre-miRNAs, regulates specification of hematopoietic progenitors to immune cell fates, as well as the development of HSPCs from embryonic stem cells. To further investigate miRNA regulation of hematopoiesis, we generated mirn23a-/- mice. These mice are viable due to compensation from the homologous mirn23b gene; however, the hypomorphic expression of mirn23a/b miRNAs results in increased B cells, and decreased monocytes/granulocytes in the bone marrow and spleen. These changes in immune cell populations appear to be due to effects on bone marrow progenitor populations. Preliminary evidence suggests that mirn23a/b miRNAs affect transcription factor expression in the multipotential progenitor (MPP) upstream of lymphoid and myeloid progenitors. Mirn23a modulates both the PI3K/ Akt pathway, and BMP/ Smad pathways, which has the potential to influence the balance of transcription factors in hematopoietic progenitors. In this project we wil examine hematopoiesis in mice lacking both the mirn23a, and mirn23b genes. We hypothesize that the mirn23a/b miRNAs act as a rheostat for cell signaling pathways, which control the development and function of adult HSPCs. The completion of this project will define the role of the mirn23a/b miRNAs in adult HSPC development and maintenance, as well as determine whether mirn23a/b expression is a novel mechanism for coordinating PI3K/ Akt, and BMP/ Smad activity during hematopoiesis. Importantly results will demonstrate whether the mirn23a/b miRNAs are regulators of hematopoietic differentiation that could be pursued as targets for cytopenia therapy. Targeting miRNAs may carry less risk of promoting malignancies than current cytokine regimens or use of cytokine mimetics. Breaking away from the standard approach of cytokine stimulation could be an innovative approach to the problem of treating cytopenias, which would be more convenient than frequent intravenous cytokine injections, and have reduced risk of harmful side effects.